1. Field of the Invention
The present invention relates to telephony communications and, more particularly, to the testing of loopback circuits using 2B1Q modulation for transmission such as found in private line data circuits.
2. Statement of the Problem
ISDN (Integrated Services Digital Network) standards have been promulgated for Basic Rate and Primary Rate transmission. The Primary Rate sets standards for the transmission of high speed T1-type signals at a transmission rate of 1.544 Mb/s. The ISDN standard for the Primary Rate is known as the 23 B+D. The ISDN standard for Basic Rate which is used for the transmission of two distinct conversations or data signals is known as 2B+D (i.e., the two voice circuits plus one data circuit).
For the Basic Rate, each conversation or data signal is delivered at a data rate of 64 Kb/s with a 16 Kb/s signaling channel. This rate corresponds to a payload data rate of 144 Kb/s with overhead information of 16 Kb/s. Hence, the full data rate for the Basic Rate is 160 Kb/s per second.
The Basic Rate 2B1Q transmission scheme utilizes four discrete voltage levels (+3, +1, -1, -3 volts). Each discrete voltage level represents two bits of information and, therefore, the 160 Kb/s data rate is actually transmitted at 80 Kb/s. The 2B1Q modulation scheme is an important improvement over traditional analog transmission schemes which are significantly affected by circuit impairments that degrade normal analog transmission such as impulse noise or cross-talk. The 2B1Q scheme utilizes integrated circuits which include transceiver circuits, ECHO cancellation, control circuitry, interface circuitry and which provides for bidirectional transmission over a single pair of wires.
Although 2B1Q technology for Basic Rate transmission represents a significant improvement over conventional analog transmission schemes, telephone companies have a substantial investment in test equipment based upon the analog transmission approach. Hence, a problem arises that a need exists for implementing the 2B1Q digital type transmission scheme in a fashion so as to retrofit with existing telephony circuits and, in particular, to enable the operating telephone companies to utilize the substantial equipment in the field based upon the analog transmission scheme such as the testing equipment for current private line data circuits of the type shown in FIG. 1.
In FIG. 1, two conventional private line data circuits 10 are shown interconnected with two channel units 80 in a conventional channel bank 20. The two private line data circuits 10 also interconnect to customer provided equipment (CPE) 30 over a network 40 such as modems 32. A point of demarcation exists between the CPE 30 and the telephone network 40. The channel unit 20 can be conventionally a D4 or an SLC 96 channel unit and is connected to conventional T1 transmission lines 50.
The arrangement shown in FIG. 1 permits the long distance connection of computers so that two computers can talk directly to each other, transfer data, and to do this with a minimal amount of errors in the transmission. In FIG. 1, each four wire equalized transmission only (ETO) channel unit 80 in the central office channel bank 20 is connected to a data station termination (DST) 12 at the customer's premise. DSTs constitute termination equipment generally referred to as network channel termination equipment (NCTE). If two data circuits B1 and B2 are required at the customer's premise, then two four wire ETO channel units and two DSTs are required. This is the configuration of FIG. 1. Each data circuit 10 requires two cable pairs 60 and 70--one pair (T1, R1) 60 for transmit and one pair (T,R) 70 for receive. In the configuration of FIG. 1, four cable pairs are required for implementation of the two separate analog data circuits 10.
Private line data circuits 10 including the actual cable pairs and the DSTs must be tested. For example, the channel units 80 and the DSTs 12 must be aligned for a one KHz level and frequency response. This alignment procedure is complicated and typically requires several persons. The alignment procedure also involves multiple sending and measuring sequences. Furthermore, the operation of these components 12 and 80 can be effected by impairments on the cable pairs 60 and 70 such as impulse noise, cross-talk or 60 Hz interference.
In FIG. 2, a first conventional approach for testing the private line data circuits 10 of FIG. 1 is shown for one circuit. Most telephone companies use remote testing 230 and remote access 240 systems to access the private line data circuit and to measure the tones appearing thereon. For example, to facilitate remote testing of the circuit, a DST 12 is often equipped with a loopback circuit 200 having relay contacts 210A, 210B and 210C. Loopback is a method of performing transmission tests of access lines from the serving switching center (i.e., the channel bank 20) that does not require the assistance of personnel at the served terminal (i.e., the NCTE). This loopback circuit 200 connects the incoming signal on the receive line 70 back to the transmit lines 60 by opening contacts 210A and 210B and closing contact 210C. Hence, an analog signal can be transmitted from the channel unit 80 to the DST 12 looped back at the DST 12, received and measured at the channel unit 80. This verifies the integrity of the entire circuit 10 from one central location. The loopback circuit 200 detects a unique tone for a predetermined period of time such as two seconds on lines 220 and then activates the contacts 210A through 210C.
In FIG. 2, the testing equipment 230 is connected to a remote access system 240 containing relay contacts 250A through 250D. The remote testing system is connected over lines 232 to the remote access system 240. Hence, when it is desired to conduct a test of the circuit, the remote testing system 230 causes the remote access system 240 to open contacts 250A and 250B and to close contacts 250C and 250D. This physically disconnects the channel unit 80 from the transmit receive lines 60 and 70 and enables the remote testing system 230 to send a testing tone on receive lines 70 to activate the loopback 200 to respectively close contact 210C and to open contacts 210A and 210B. The remote testing system 230 can then generate a conventional predetermined pattern of analog signals to thoroughly test the entire circuit. Upon completion of the test, the loopback circuit 200 is deactivated thereby closing contacts 210A and 210B and opening contacts 210C and causing the remote access system 240 to close contacts 250A and 250B and to open contacts 250C and 250D. FIG. 2 illustrates the loopback for the B1 circuit, the second B2 circuit would operate in the same fashion as a separate loopback.
A second conventional loopback test can be originated from a distant location such as at a central office or other central location. A digital signal corresponding to the required loopback tone is delivered on the T1 carrier, converted into analog signal on the T, R, T1, R1 leads for delivery into the DST. Loopback is detected and relay 200 activated so as to perform the remaining tests.
It is also to be expressly understood that a number of configurations exist in the telephone company environment and while FIGS. 1 and 2 show two private line data circuits each having four wire channels, it is to be expressly understood that one or more data circuits could be provided to a customer and that two wire circuits could be utilized. It is also to be expressly understood that a loopback could be provided in the channel unit and that the T1 carrier and channel unit can be tested from the distant location.
Furthermore, most telephone companies utilize testing systems such as a switched maintenance access system (SMAS) or a switched access system (SAS) as the remote access system 240 and utilize a switched access remote test system (SARTS) for the remote testing system 230. These are conventional access and testing systems and are well known in the telephony art. The remote access systems 240 connect metallically to the tip and ring leads of pairs 60 and 70 at the channel unit 80. Additionally, as shown in FIG. 2, E and M leads 280 are also provided to the channel units 80. These E and M leads 280 are available in the channel unit as the transmit and receive leads for signaling. However, the E and M leads are not used as part of the remote testing system 230 or the remote access system 240. Conventionally, the six leads of the channel unit 80 (i.e., T, T1, R, R1, E and M) are typically wired out to a manual crossconnect frame (not shown).
A need, therefore, exists to utilize existing telephone company remote testing systems 230 and remote access systems 240 such as SMAS, SAS, and SARTS and/or conventional distant loopback tests, as a telephone company implements the 2B1Q modulation transmission scheme between NCTEs such as DSTs and channel units 80 or for testing loopbacks in channel units.
3. Solution to the Problem
The present invention provides a solution to the problem through a unique implementation of the 2B1Q modulation technology such as between conventional NCTE such as DSTS and channel units so that conventional remote testing and access system equipment and/or distant testing equipment can be fully utilized.